Conventionally, the Long Term Evolution (LTE) system is known as a network system for wireless communication between a base station and a terminal apparatus. The LTE system is a mobile communication system of next generation evolved from the Universal Mobile Telecommunications System (UMTS) and is intended to provide more improved mobile communication services.
In the LTE, in addition to a macro cell wireless communication base station apparatus (Evolved NodeB, or eNB) that is primarily located outdoors and supports a larger cell diameter, it is under consideration to provide a femto cell wireless communication base station apparatus, which supports a cell diameter on the order of tens of meters, in indoor facilities such as a residence, an office, a restaurant, and a shopping center. Some femto cell wireless communication base station apparatuses under consideration in the 3rd Generation Partnership Project (3GPP) permit limited groups of members to access the base station apparatuses. Such cells are referred to as a Closed Subscriber Group cell, or CSG cell. A wireless communication base station apparatus that forms a CSG cell is referred to as a Home Evoled NodeB, or HeNB. The macro eNB can, for example, manage two or more different frequency bands (f1, f2, . . . , fn), and the home eNB may be located on a frequency f1.
A wireless communication terminal apparatus (User Equipment, or UE) permitted to have an access is controlled from the network so as to connect to the home eNB. Specifically, once in the area of the CSG cell, the terminal apparatus is controlled to preferentially connect to the home eNB even if the terminal apparatus can receive radio signals from the eNB. A list of CSG cells that the terminal is permitted to access is referred to as a whitelist. Each of terminal apparatuses has its own whitelist.
Description will now be made to a control method of performing a handover from a macro eNB to a home eNB in a conventional LTE. (refer to, for example, Patent Literature 1 and Non-Patent Literature 1)
An active UE generally receives a measurement control message through a Dedicated Control Channel, or DCCH, from the eNB to which the UE is connected. The measurement control message provides settings for taking reception quality measurements of the current cell and an adjacent cell. The measurement control message includes parameters such as a frequency and a system to be measured, event information that triggers transmission of a measurement report to the base station, a signal used in the measurement, and a duration (gap) for performing the measurement. The UE measures reception quality of the adjacent cell through a pilot channel (Common Pilot Channel, or CPICH) according to the settings from the measurement control message, and transmits the reception quality measurement result in a report (measurement report) to the eNB in a periodical manner or triggered by a configured event.
It is assumed here that the UE moves to the vicinity of the border of the CSG cell. The UE measures the reception quality of the CSG cell, during which the UE acquires a physical cell ID (Physical Cell Identity, or PCI) of the CSG cell by receiving a Synchronization Channel, or SCH. In particular, one PCI is determined among 510 PCIs by a combination of a signal pattern of a Primary Synchronization Channel (P-SCH) and a pattern of a pair of (S1, S2) or (S2, S1) determined from an Secondary Synchronization Channel (S-SCH). The UE then notifies the macro eNB of the PCI of the CSG cell subjected to the measurement and the measurement result.
The macro eNB retains a list of CSG cells contained in the macro cell, and the list of CSG cells include PCIs for each home eNB and a Cell Global Identity, or CGI. The macro eNB that has received the measurement result of the CSG cell notified by the UE determines a home eNB that has the corresponding PCI from the list of CSG cells contained in the macro cell, and transmits an inquiry (HO request) of whether the UE may be allowed to perform a handover via a Mobility Management Entity (MME)/Gateway (GW) to the home eNB. If the handover of the UE is to be permitted; the home eNB that has received the HO request reserves in advance wireless resources appropriate for services provided to the UE, performs acceptance control, and transmits a response (an ACK to the HO request) that permits the handover of the UE via the MME/GW to the macro eNB.
Upon receiving the ACK to the HO request from the home eNB, the macro eNB transmits an instruction (HO command) causing a transfer to the CSG cell to the UE, and the UE transmits a random access preamble (RACH preamble) in order to establish synchronization with the specified home eNB. Thereafter, when the UE receives a response (random access response, or RACH response) from the home eNB, the UE establishes synchronization with the home eNB and an uplink transmission opportunity is assigned to the UE from the home eNB. Once the synchronization is established, the UE transmits a signal (HO confirmation) indicative of the completion of handover to the home eNB.